Colloidal Synthesis of Lead-Free Silver-Indium Double-Perovskite

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C: Physical Processes in Nanomaterials and Nanostructures

Colloidal Synthesis of Lead-Free Silver-Indium Double-Perovskite CsAgInCl Nanocrystals and Their Doping with Lanthanide Ions 2

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Wonseok Lee, Seunghwa Hong, and Sungjee Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12146 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

Colloidal Synthesis of Lead-Free Silver-Indium Double-Perovskite Cs2AgInCl6 Nanocrystals and Their Doping with Lanthanide Ions Wonseok Lee,1,† Seunghwa Hong,1,† and Sungjee Kim1,2,*

1Department

of Chemistry, Pohang University of Science and Technology (POSTECH),

77 Cheongam-ro, Nam-gu, Pohang 37673, South Korea

2School

of Interdisciplinary Bioscience and Bioengineering, Pohang University of

Science and Technology, 77 Cheongam-ro, Nam-gu, Pohang 37673, South Korea

AUTHOR INFORMATION

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT Recently, lead halide perovskite (LHP) materials have proven the potential for many optoelectronic applications. However, the structural instability and toxicity concerns necessitated robust and heavy-metal-free alternatives. We report synthetic route to Cs2AgInCl6 double-perovskite nanocrystals (DP NCs) that have the crystal structure replacing two lead ions in LHP by a pair of silver and indium ions. Cubicshaped Cs2AgInCl6 DP NCs were obtained with the narrow size distribution, which showed superior stability against moisture that rivalled core-shell structured CdS/ZnS NCs. Cs2AgInCl6 DP NCs showed discrete optical properties: (i) multiple absorbing states that resolved the parity-forbidden band edge transition at 3.33 eV and nextfollowing higher order parity-allowed transition at 4.88 eV, and (ii) multiple emission states that consisted of the band edge emission at 350 nm and emissions from


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defective states peaked at 395 nm. The synthetic route was further exploited for isovalent doping with lanthanide ions, which yielded Yb-doped, Er-doped, and Yb/Er codoped Cs2AgInCl6 DP NCs. The characteristic f-f transition emissions were observed in infrared

at

996

nm

for

Yb3+

and

at

1537

nm

for

Er3+

dopants.

Introduction


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Lead halide perovskites (LHPs) have attracted great interest for many applications that include solar cells,1,2 light emitting diodes,3-5 lasing,6,7 X-ray detectors and scintillators.8-10 They show unique properties such as high carrier mobility11 that rise from the ‘defect tolerant’ electronic structures.12,13 Synthesis of LHP colloidal nanocrystals (NCs) has been also fiercely studied not only for an alternative to the precursor-based solution fabrication process but also for a novel class of nanofluorophores.14 Regardless of the size, LHPs typically suffer from the limited stability against moisture and heat.15,16 Potential toxicity concern from the heavy metal lead ions also hurdles the realizations of many applications using LHPs.15,16

To overcome the challenges, it is highly sought for to synthesize stable and environmentally friendly perovskites of heavy metal free compositions. A promising strategy to obtain such alternative perovskites is to replace lead ions in LHPs with other proper metal ions. The lead-replacing metal ions should fit for the size and charge to retain the 3D perovskite crystal structure and not fragmentalize into 0D, 1D or 2D perovskite derivatives.17,18 In addition, it is desired to have similar electronic structures


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as LHPs that exhibit the ‘defect tolerant’ properties, which majorly attribute to the dispersive valence band structure and antibonding orbital character at the valence band edge. Simulation studies have predicted that ions of a lone 6s2 or 5s2 pair of electrons that give rise to proper s and p orbital levels and the spin-orbit coupling can replace lead ions and form the ‘defect tolerant’ electronic structures.13 Promising candidate ions include partially oxidized, heavy post transition metal ions such as In+, Tl+, Sn2+, Sb3+, and Bi3+.

In terms of the valency, lead ions can be replaced by isovalent ions or by combinations of heterovalent ions that match the charge neutrality. It may be more straightforward to substitute Pb2+ ions with other carbon group ions such as Ge2+ or Sn2+. There have been a few reports on the synthesis and applications of tin halide based perovskites.19,20 Unfortunately, tin halide perovskites showed structural instability due to the multiple valencies of tin ions which allowed facile oxidations to Sn4+ states. On the other hand, replacing two Pb2+ ions with one monovalent and one trivalent ions and building so called double-perovskite (DP) structures showed promising results.21,22


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DPs have elpasolite structure: A2M+M3+X6 where A = Cs, CH3NH3; M+ = Ag, Au, Tl; M3+ = In, Tl, Au, Bi, Sb; X = Cl, Br, I. Discovery of elpasolite DP structures such as Cs2NaFeCl6 and Cs2NaLaCl6 dates back to the end of 1960s and early 1970s.23 Recently, synthesis of elpasolite Cs2AgBiX6 DP was reported by a few groups. Bulk Cs2AgBiBr6 and Cs2AgBiCl6 showed the indirect bandgaps.21,22 Bulk Cs2AgBiBr6 showed the photoluminescence (PL) at 1.87 eV that was assigned as surface-related emissions.21,22 Synthesis of Cs2AgBiX6 DP NCs was reported more recently by a few groups and the Cs2AgBiX6 DP NCs showed the bulk-like surface emissions.24-27 Cs2AgBiX6 DP NCs showed the low PL quantum yields (QYs) and broad absorption/emission spectra when compared to those of typical LHP NCs, which may originate from their indirect bandgap characteristics.21,22 Though limited by the indirect bandgap characteristics, both bulk Cs2AgBiX6 DPs and Cs2AgBiX6 DP NCs showed superior structural stability than conventional LHPs.21,22,24,25 On the other hand, Cs2AgInCl6 DP was predicted to have a direct bandgap by computational studies,28 and a direct bandgap Eg of 3.2−3.3 eV was determined from bulk Cs2AgInCl6 crystals prepared by a hydrothermal method.29-32 The Cs2AgInCl6 crystals also showed robust


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stability against moisture and heat, enduring up to 400 °C without noticeable decompositions.29

The direct bandgap characteristics and structural robustness make Cs2AgInCl6 DP NCs an ideal lead-free alternative for LHP NCs. Gamelin and co-workers have reported one-pot colloidal synthesis of silver-bismuth DP NCs (Cs2AgBiX6) using TMS-X (TMS = trimethylsilyl) as the halogen precursor.24 Inspired by this pioneering work, we first tried to synthesize Cs2AgInCl6 DP NCs by replacing the bismuth by indium precursor, which only resulted in mixtures of various nanoparticles (NPs) of multiple compositions that include silver-containing secondary phases. Synthetic parameters such as precursor ratio between Cs, In, Ag, and Cl were optimized to yield cubic-shaped narrowly sizedistributed Cs2AgInCl6 DP NCs. The obtained NCs showed distinctive optical transitions both in absorption and emission. We have further utilized the synthetic route for lanthanide-doped Cs2AgInCl6 DP NCs. Recently, many synthetic strategies have been developed to introduce dopants in semiconductor NCs.33 The strategies include use of co-precursors during NC growth,34 and cation exchange of pre-prepared NCs.35 Yb-


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doped, Er-doped, and Yb/Er co-doped Cs2AgInCl6 DP NCs were synthesized by additionally introducing Yb and/or Er acetate salts. The doped Cs2AgInCl6 DP NCs retained the size-distribution and structural integrity within our doping levels and showed the infrared (IR) emissions from the partially filled 4f orbital energy levels of the lanthanide ions.

Considered to be quite parallel to our efforts, colloidal synthesis of Cs2AgInCl6 DP NCs has been very recently reported by De Trizio, Manna, and co-workers.36 They have previously reported an improved synthetic route for both all-inorganic and hybrid organic–inorganic LHP NCs that can sophisticatedly control the precursor stoichiometry as decoupling the halogen origins by using lead salt and benzoyl halide precursors.37 They have further extended the strategy to yield Cs2AgInCl6 DP NCs and Mn-doped Cs2AgInCl6 DP NCs. We were inspired by the precursor decoupling idea, and have used halide-free metal salts for Cs, Ag, and In precursors along with TMS-Cl chlorine precursor to synthesize Cs2AgInCl6 DP NCs. Under transmission electron microscope (TEM), our NCs looked quite alike to those by De Trizio, Manna, and co-workers.


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However, our Cs2AgInCl6 DP NCs showed multiple optical transitions that are assigned as parity-forbidden band edge transition and allowed transitions of higher orders. We believe the bandgap of our 10.6 nm Cs2AgInCl6 DP NCs to be 3.33 eV, which is more than 1.0 eV smaller than the bandgap value reported for 9.8 nm Cs2AgInCl6 DP NCs reported by De Trizio, Manna, and co-workers. We also observed the resolved emission that consisted of band edge and trap-related red-shifted emissions. The band edge emission was not observed for those by De Trizio, Manna, and co-workers. Both Cs2AgInCl6 DP NC samples prepared by us and De Trizio, Manna, and co-workers seem to share the same crystal structure as determined by the X-ray diffraction (XRD) and similar size and shape. However, their surface and trap states may be quite different as resulted from the disparities in the synthetic protocols that include reaction temperature (105 °C vs. 200 °C), halide precursor (benzoyl halide vs. TMS-X), and precursor stoichiometry. The two Cs2AgInCl6 DP NC samples showed notable difference in the optical properties presumably because of the different surface states and also possibly by different internal traps such as anti-sites (exchange between Ag and In sites).29


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The band structure of Cs2AgInCl6 is still somewhat controversial. DP materials have the centrosymmetric crystal structure. Their transition between valence band maxima (VBM) and conduction band minima (CBM) can be parity-forbidden, and so is the case for Cs2AgInCl6.38 Cs2AgInCl6 has the direct bandgap with both VBM and CBM positioned at the Γ point. The band edge transition is forbidden by the Laporte rule and is expected to be weak. There are neighbouring higher states above VBM at the Γ point. The transitions from the VBM and from the next higher VB state to CBM are both parityforbidden, but the second higher VB state is expected to have the different parity from that of CBM and the transition should be parity-allowed (Scheme 1).29 Optical measurements on bulk crystal Cs2AgInCl6 consistently reported the absorption transition at 3.2−3.3 eV and PL peak near 2.0 eV.31 Based on the apparently large Stokes shift, some researchers have argued the absorption as the allowed transition to higher order states and the PL as parity-forbidden lowest energy transition between VBM and CBM.32,38 To the contrary, other researchers assigned the absorption as the optical bandgap corresponding to the parity-forbidden lowest energy transition between VBM and CBM and the PL as the emission arisen from surface or defective states.29-31 We


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believe that our Cs2AgInCl6 DP NCs showed the band edge emission from the parityforbidden transition between the CBM and VBM. The band edge emission was never reported from the bulk Cs2AgInCl6. Our Cs2AgInCl6 DP NCs may have a well-passivated surface state that allowed the observation of the band edge emission. Compared to the bulk, the small NCs may be less susceptible to the defect emissions such as from the anti-sites presumably because of the facile self-repair of the anti-sites by site exchanges of the cations within the nanoscale crystal framework.

Scheme 1. (Left) Predicted electronic structure of Cs2AgInCl6, (right) transition energies observed from our Cs2AgInCl6 DP NCs and calculated transition energies for bulk Cs2AgInCl6.27


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We have studied stability of our Cs2AgInCl6 DP NCs against moistures to verify if the enhanced stability reported for bulk Cs2AgInCl6 DP can be extended to our NCs.29 Our Cs2AgInCl6 DP NCs showed exceptional robustness against moistures, which was comparable to that of core-shell structured II-VI semiconductor NCs. Core/shell CdS/ZnS NCs were used as the control. Our Cs2AgInCl6 DP NCs retained the PL intensity for days in the bilayer system with direct contact to water, showing the contrast larger by orders of magnitudes when compared to CsPbBr3 NCs.

Doping impurity ions into colloidal NCs can endow new properties that include optical, electronic and spintronic functionalities. Lanthanide-doped materials such as Er-doped and Yb/Er co-doped materials are prototypes for many telecommunication applications and proper host materials that can augment the weak absorption of the lanthanides have attracted great interest.39 Lanthanide-doped perovskite NCs can be a new class material for optical communications such as a key component in waveguide amplifiers. Recently, Song and co-workers reported lanthanide doping in CsPbX3 NCs.40,41 The lanthanide-doped NCs exhibited sharp emissions that correspond to the transitions


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between partially filled 4f orbital energy levels. Surprisingly, PL QY exceeding 100% was reported from Yb-doped CsPbX3 NCs. Gamelin and co-workers have studied excitonic energy transfers to Yb3+ dopants in CsPbCl3 NCs. The excitonic energy transfers are expected to be inefficient because of the weak exchange interaction between the lanthanide f orbital and the perovskite excitons. The high lanthanide emission QY from the doped perovskite NCs is intriguing. It was hypothesized that heterovalent Yb3+ dopants in LHP NCs formed Yb3+-VPb-Yb3+ defect complex which facilitated the quantum cutting, sharing excitation energy to two neighbouring Yb3+ ions.42 So far, isovalent lanthanide doping in perovskite NCs has not been reported. DPs that have both monovalent and trivalent metal ions can provide unique opportunities to introduce lanthanide as a isovalent fashion. We have isovalently introduced Yb3+ and Er3+ in replacement of In3+ to Cs2AgInCl6 DP NCs and successfully tuned the emission properties to infrared. We have prepared Yb-doped, Er-doped, and Yb/Er co-doped Cs2AgInCl6 DP NCs and observed the dopant emissions at 996 nm and/or 1537 nm. Isoovalently Yb-doped NCs showed the significantly lower dopant PL QY than that reported for heterovalently Yb-doped LHP NCs. We suspect defective


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sites generated by the heterovalent doping in Yb-doped LHP NCs acted as fast energy relay sites to the neighbouring dopant sites.

Results and Discussion

Our Cs2AgInCl6 NCs were synthesized by solvothermal method. In brief, 2:1:4 mole ratio of cesium carbonate (Cs2CO3), silver acetate (Ag(OAc)), and indium(III) acetate (In(OAc)3) were suspended in a mixture of 1-octadecene (ODE), oleic acid (OA), and oleylamine (OAm) and degassed under vacuum for 2 hours at 100 °C (see Supporting Information for details). The solution was then heated to 200 °C under N2 atmosphere, and 18 mole equivalent of chlorotrimethylsilane (TMS-Cl) to indium in ODE solution was swiftly injected. After 2 minutes the reaction mixture was cooled down to room temperature, and the Cs2AgInCl6 NC products were separated by centrifugation. Our representative Cs2AgInCl6 DP NCs showed cubic morphology of the average 10.6 nm edge length with the size distribution of 10.4% (Figures 1a and S1a). Each NC was highly crystalline under high magnification TEM. Figure 1b shows (220) lattice planes lying along the diagonal of a NC cube face. The lattice fringe d-spacing was 0.38 nm,


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which accords well with known bulk lattice parameter of Cs2AgInCl6 (a = 1.048 nm) (Figure 1b).29 XRD patterns of NCs matched well with those from the bulk, which confirms the absence of secondary crystalline phases such as silver-containing compositions like AgCl or metallic Ag (Figure 1c). The NCs have cubic diffraction pattern with a Fm-3m space group that is analogous to the cubic LHP materials. Elemental analysis by STEM-EDS returned the Cs:Ag:In:Cl stoichiometry as 2.0:1.2:1.5:6.4, which slightly deviates from the bulk composition ratio of 2:1:1:6 (Table 1). EDS mapping also showed spatially homogeneous co-existence for all the composing elements, which corroborates the absence of secondary phases (Figure S2).


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Figure 1. (a, b) TEM images for the representative Cs2AgInCl6 DP NCs. (c) XRD pattern of the NC powder (top panel) and reported pattern of bulk Cs2AgInCl6 (bottom panel). (d) The NC absorption spectrum in hexanes solution, showing the strong absorption at 240 nm. The inset is the absorption spectrum measured using a more concentrated solution of the same NC to reveal the weak band edge transition. (e) Tauc plot for the strong transition in the NC absorption. The inset shows the Tauc plot for the weak band edge transition from the concentrated NC solution. (f) Absorption, PL and PLE spectra of the NC sample.


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element

atom [%]

std [%]

Cs

18.1

0.5

Ag

10.5

0.3

In

13.8

1.1

Cl

57.6

1.3

Table 1. Relative molar compositions of Cs2AgInCl6 DP NCs determined by STEM-EDS Figure 1d shows the absorption spectrum of Cs2AgInCl6 DP NC solution in hexanes, which exhibits a strong peak at 240 nm with the tail reaching close to 400 nm. Inset of Figure 1d shows the absorption spectrum near the absorption onset (300~400 nm), which was measured using the same NC solution of ~200 times higher concentration. A rather broad peak which was hidden for the dilute NC spectrum was revealed at around 300 nm. We have assigned the broad peak as the optical bandgap (parity-forbidden transition from VBM to CBM, illustrated as transition I in Scheme 1) and the strong peak as the transition between third VB level and CBM (parity-allowed transition III in Scheme


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1). We presume the weak transition II was overlapped by other transitions and not resolved. The extinction coefficient at 300 nm was 118 times smaller than that at 240 nm, which indicates forbidden and allowed transition nature of transition I and III. Tauc plots were employed using the exponents of 2/3 and 2 respectively for the transition I and III (Figure 1e). The Tauc plots returned the optical bandgap (transition I) of 3.33 eV which coincides well with the measured and calculated bulk bandgaps by others (Scheme 1).29 For the transition III, 4.88 eV was obtained by the Tauc plot (inset in Figure 1e), which also accorded well with the calculated bulk value of 4.46 eV (Scheme 1).29 Optical transitions from the direct and parity-forbidden band edge and following higher order parity-allowed states have been also reported for Cu2O, which shares the absorption profile as ours showing weak band edge tail and strong peaks at higher energies.43 The NC sample showed the broad emission with the peak at 395 nm and a resolved hump at 350 nm (Figure 1f). The emission PL QY was 0.5 ± 0.3%. To further understand the emission states, PL excitation (PLE) spectra were obtained using the emission wavelengths at the blue edge of the 350 nm hump peak (at 330 nm, as


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indicated by the blue arrow in Figure 1f) and also at the red edge of the 395 nm main peak (at 430 nm, as by the green arrow in Figure 1f) to minimize the interference from the neighbouring emission states (Figure 1f). Both PLE spectra commonly have the sharp peak at 300 nm, which is considered to be a band edge related emission. The broad PLE peak at 345 nm is attributed to defect or surface related sub-bandgap states. PLE measurements using the emission wavelengths at the peak positions were also performed, which gave similar but less resolved spectrographs (Figure S3). Synthesis of quaternary compounds such as Cs2AgInCl6 typically requires careful stoichiometric consideration between the composing elements to avoid undesired creations of secondary phases. For an example, secondary phases such as AgX, CsX, Cs-Ag-X and Cs-Bi-X can be inadvertently obtained in trials of Cs2AgBiX6 DP NC synthesis.24,27 For our Cs2AgInCl6 DP NC preparations, metallic salts of Cs, Ag, and In were activated using oleylamine and oleic acid and TMS-Cl was injected for nucleation and subsequent growth into NCs (optimal precursor feed ratio of Cs:Ag:In:Cl = 4:1:4:72 at 200 °C). To optimize the NC yield and avoid the secondary phases, we have studied the effects of various synthetic parameters: halogen precursor content over Ag and In


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precursors, reaction temperature, and In to Ag precursor ratio (Figures 2 and S4). Excess chlorine precursor injection (Cl to Ag and In = 9) yielded uniformly shaped and narrowly size-distributed NCs under TEM (Figures 2a and 2b). Stoichiometric (Cl to Ag and In = 3) or less amounts of chlorine precursor resulted in additional spherical 2~3 nm NPs (red box in Figure 2a and also see enlarged TEM image of Figure S4) that exhibited a broad peak in the absorption spectrum at around 400~500 nm (Figure S5a), which may attribute to silver-containing secondary phases.24,27,36 Reaction temperature of 200 °C was optimal for uniform sized cubic NCs (Figures 2c, 2d and S5b). Lower reaction temperatures typically yielded mixtures of cubic and irregularly shaped NPs. Higher reaction temperatures resulted in low reaction yields presumably due to the volatility of the chlorine precursor. In to Ag precursor of 4 was chosen to avoid spherical or amorphical nanostructures of secondary phases (Figures 2e, 2f, and S5c). In the case of lower In to Ag precursor ratio than 4, additional broad absorption band was observed.


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Figure 2. TEM images of NP products obtained by different reaction conditions. (a, b) Effect of TMS-Cl to (Ag and In) molar ratio, Cl/M+M3+: (a) Cl/M+M3+ = 3 and (b) Cl/M+M3+ = 9. The red box in (a) shows the region where 2~3 nm sized silver-containing secondary phase NPs can be found. (c, d) Effect of reaction temperature: (c) at reaction temperature of 170 °C and (d) at reaction temperature of 200 °C. (e, f) Effect of In to Ag molar ratio: (e) In/Ag = 2 and (f) In/Ag = 4. Detailed reaction conditions can be found in the Supporting Information (Table S1).

Bulk Cs2AgInCl6 DP showed superior stability against heat and moisture over conventional LHPs.29 We have tested the stability of our Cs2AgInCl6 DP NCs to see if


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the enhanced stability of DP over LHP can be extended to our NCs (Figure 3). In addition to Cs2AgInCl6 DP NCs and CsPbBr3 NCs, II-VI semiconductor NCs of CdS NCs and CdS/ZnS core/shell NCs were also used as controls. II-VI semiconductor NCs are known to be more robust than conventional perovskite NCs. Core/shell type NCs such as CdS/ZnS core/shell NCs should be very robust because of the passivation from the inorganic shell material of large band gap. All the four NC samples (10.6 nm Cs2AgInCl6 DP NCs, 8.6 nm CsPbBr3 NCs, 3.6 nm CdS NCs, and 5.3 nm CdS/ZnS core/shell) were dissolved in ODE (See Figure S6 for the absorption spectra, emission spectra, and TEM images of the four NC samples). Bilayers were made by placing the NC samples on top of excess deionized water. The bilayer system exposes the NCs to direct contact to water, and it was chosen to test the stability against moisture. To our surprise, Cs2AgInCl6 NCs showed the stability comparable to that of core-shell structured CdS/ZnS NCs. CsPbBr3 NCs showed the weakest stability. The PL intensity was halved in 3.5 hrs and almost quenched within a day. The bare CdS NCs were followed by showing 26% PL diminishment in two days. CdS/ZnS NCs and Cs2AgInCl6 DP NCs respectively showed 12% and 6% decrease in PL intensity in two days but within the


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same statistical range. Our Cs2AgInCl6 DP NC was as stable as II-VI core/shell NCs, which may attribute to the low water solubility of silver and indium halide salts.

Figure 3. PL stability test of four different NC samples. Time evolutions of PL intensities of the four samples of Cs2AgInCl6 DP NCs (black), CsPbBr3 NCs (red), CdS/ZnS core/shell NCs (blue) and CdS NCs (green) in bilayer system of direct contact to water. All data points are normalized by the PL intensity at their initial values.

Our synthetic route to Cs2AgInCl6 DP NCs allowed simple introduction of heterogeneous metal ions by co-adding the precursors in the reaction pot. Additions of 10% and 20% of Yb to In yielded Cs2AgInCl6 DP NCs doped with 0.6% and 0.9% of Yb3+ dopants, respectively, as determined by inductively coupled plasma mass spectrometry (ICP-MS). Both samples showed similar absorption spectra as that of


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undoped Cs2AgInCl6 DP NCs (Figure 4a). A sharp PL peak at 996 nm was observed upon the excitation at 300 nm, which originates from f-band transition of Yb (2F5/2 to 2F ). 7/2

The emission peak was significantly red-shifted from typical Yb peaks from Yb-

doped oxide or fluoride NCs (typically emit near 970−980 nm), presumably due to nephelauxetic effect that decreased the f-band transition energy in the highly polarizable lattice.44,45 The dopant PL QY increased from 1.8 ± 0.5% to 3.6 ± 0.4% as varying the doping level from 0.6% to 0.9%. Use of higher feed ratios of Yb/In such as 30% in synthesis resulted in co-production of undesired secondary phases. Similarly, Er-doped Cs2AgInCl6 DP NCs were prepared using a feed ratio of Er/In = 20%, which yielded 2.2% doped NCs. Er-doped NCs showed sharp PL band peaked at 1537 nm that originates from f-band transition of Er (4I13/2 to 4I15/2). The PL QY was 0.05 ± 0.01%. Both Ybdoped NCs and Er-doped NCs showed highly crystalline and cubic shaped structure in TEM images (Figures 4b and 4c). The size distribution of the Yb-doped NCs and Erdoped NCs were 8.3 ± 1.0 nm and 7.2 ± 0.7 nm, respectively (Figures S1c and S1d). Both doped NCs showed the XRD patterns same as undoped Cs2AgInCl6 DP NCs, which indicates that their crystal structures were not distorted by the doping (Figures 4d


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and 4e). High-resolution TEM images revealed the lattice fringes similar to what observed for the undoped NCs (Figure S7). The absorption spectra of doped NCs were slightly broadened presumably due to minor alterations in the electronic structures (Figures 4f and 4g). We hypothesize that partial contributions of 6s orbitals from Yb or Er in the CBM along with 5s orbitals from In can complicate the band structures. PLE spectra of doped NCs showed a sharp peak around ~300 nm which is assigned for the band edge absorption. Undoped NCs showed two peaks: one assigned as the band edge related emission and the other corresponding to the surface and/or defective states. To the contrary, Yb-doped NCs and Er-doped NCs showed only the band edge peak and did not show the defective states related peak at 345 nm. Excitons trapped in the defective states in NCs are considered incapable of transferring energies to the lanthanide dopants, which resulted in the absence of the defective state PLE peak for the doped NCs.


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Figure 4. (a) Absorption and emission spectra of Cs2AgInCl6 DP NCs doped with 0.6% Yb (black) or doped with 0.9% Yb (red). TEM images and XRD patterns for 0.9% Yb-


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doped Cs2AgInCl6 DP NCs (b, d) and 2.2% Er-doped Cs2AgInCl6 DP NCs (c, e). Absorption, PL, and PLE spectra for 0.9% Yb-doped Cs2AgInCl6 DP NCs (f) and for 2.2% Er-doped Cs2AgInCl6 DP NCs (g).

Our Cs2AgInCl6 DP NCs have both monovalent and trivalent metal ion sites, and the structure offers unique opportunities for isovalent cation exchanges of trivalent ions such as lanthanide ions. In the case of heterovalent cation exchanges using lanthanide ions such as Yb in LHP NCs, PL QYs exceeding 100% were reported. The high QY is quite unexpected because excitonic energy transfers should be inefficient due to the weak exchange interactions. The heterovalent cation exchange results in charge inneutrality, and the resultant defect sites such as Yb3+-VPb-Yb3+ complex is hypothesized to have facilitated the quantum cutting.42 We have isovalently introduced Yb3+ and/or Er3+ in replacement of In3+ to Cs2AgInCl6 DP NCs, which can keep the charge neutrality. As expected, our doped Cs2AgInCl6 DP NCs showed relatively weak dopant PL emissions. It is hypothesized that the exciton funnelling to Yb3+-VPb-Yb3+ complex sites in the heterovalently-lanthanide-introduced LHP NCs is much faster than


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exciton trapping to surface and/or defective states in perovskite NCs. We hypothesize that the Pb vacant site, VPb, acted as a hole trap that relayed the excitonic energy to the Yb dopants which sat right next to it via Dexter-type transfers. Our recent studies on codoped NCs revealed that a defect site created by heterovalent cation-exchanges can act as an energy transfer mediator to neighboring isovalently-introduced Mn dopants.46 The relaying defect sites resulted in sensitized Mn dopant emissions. We have not extensively optimized the synthetic conditions to maximize the dopant emission QYs. The dopant emission QYs were lower by more than an order of magnitude when compared to the heterovalently-lanthanide-introduced LHP cases. It is also noted that our DP NCs exhibited the PL QYs similar to those for isovalently Er-doped NaYF4 NCs (PL QY ~0.01−0.2%).47 We have further extended the doping strategy to synthesize Yb/Er co-doped Cs2AgInCl6 NCs (Figure 5). Additions of 10% of Yb and Er precursors to that of In yielded Cs2AgInCl6 DP NCs doped with 0.5% Yb3+ and 0.5% of Er3+ dopants. PLs from both dopants were observed: Yb from 996 nm and Er from 1537 nm. PL QYs of each dopant emission was 0.2% and 0.02%, respectively for Yb and Er. The absorption


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spectrum of co-doped NCs was also slightly broadened. Yb-doped, Er-doped and Yb/Er co-doped Cs2AgInCl6 NCs have very highly ordered crystallinity which suggests that the broadening of the absorption spectra of doped Cs2AgInCl6 NCs were mostly originated from electronic alterations by dopants rather than structural deformations.

Figure 5. Absorption (a), PL spectra (b), and TEM image (c) of Yb/Er co-doped Cs2AgInCl6 DP NCs.

Conclusions In conclusion, we have synthesized colloidal Cs2AgInCl6 DP NCs with controlled shape and size with the relative size distribution of 10.4%. The precursor feed ratio of Cs:Ag:In:Cl = 4:1:4:72 and reaction temperature of 200 °C was found optimal for our synthetic route. Our Cs2AgInCl6 DP NCs exhibited resolved absorption showing the parity-forbidden transition from VBM to CBM at 3.33 eV and parity-allowed higher order


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transition between the third VB level and CBM at 4.88 eV. The emission states were also resolved as band edge emission at 350 nm and emissions from defective states peaked at 395 nm. Our Cs2AgInCl6 DP NCs showed the superior stability against moisture comparable to that of CdS/ZnS core/shell NCs. Yb-doped, Er-doped, and Yb/Er co-doped Cs2AgInCl6 DP NCs were also prepared, and the characteristic f-f transition emissions were observed in IR at 996 nm for Yb and at 1537 nm for Er. With the enhanced stability and wavelength-tunability to the lanthanide IR emissions, Cs2AgInCl6 DP NCs are expected to open a new avenue for many optoelectronic applications in display, light source, and telecommunication networks.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge.


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Experimental methods, optical characterization data and size histograms. TEM images and STEM-EDS maps. Absorption/emission spectra and TEM images of CdS NCs, CdS/ZnS core/shell NCs and CsPbBr3 NCs. (file type, i.e., PDF)

AUTHOR INFORMATION

Author Contributions †The authors contributed equally

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2015M3C1A3056411). We also acknowledge the partial support from Pohang Iron and Steel Company (POSCO). REFERENCES


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